The present invention relates generally to beam emission control means of either a reflective or transmissive nature, and more particularly to such means as may be utilized in projection screens and the like.
Previous projection screens typically have utilized small, randomly oriented diffusive, reflective or refractive particles embedded in or on the screen to reflect or refract the image incident thereon to the observers. Such screens provide poor gain relative to ambient light, because a considerable portion of the incident light is randomly directed to points outside of the field of observation or is absorbed in the screen surface. For this reason such screens are capable of practical application only under conditions substantially free of extraneous light.
In Mihalakis U.S. Pat. Nos. 2,804,801 ('801) and 2,984,152 ('152), a projection screen was disclosed whereby substantial boundary control of the reflected light could be achieved, while at the same time substantially excluding extraneous light, thereby considerably increasing the quantum of incident light comprising the image reflected to the field of observation and thereby enhancing the contrast of the reflected image relative to surrounding lighting conditions. The screen comprised an essentially corrugated surface of high specularity, having a curvature undulating from concave to convex lengthwise of the corrugations. Each undulation was of such small size that the image reflected from each undulation appeared to a viewer to be essentially a point source of the image impinging thereon, and the aggregation of all such point source images provided a composite of the image projected on the entire screen. Because of the undulating nature of the specular screen surface, substantially all the light impinging on the screen from a "projection zone" was returned to a field of observation defined by the geometry of the undulations; that is, the screen provided containment of the impinging image within a predetermined zone of reception. At the same time, a substantial amount of the extraneous light impinging on the screen from outside the projection zone was reflected away from the field of observation. The combined result of these effects was to provide a screen with improved contrast with respect to ambient light.
While the projection screen disclosed in the '801 and '152 patents represented a substantial advance over earlier conventional projection screens, that screen did have some limitations. Reduced to its simplest components, the screen consisted of four basic elements: a first concave surface, a first convex surface, and two portions having both concave and convex surfaces. More particularly, the wholly concave and wholly convex elements provided far less than optimum image-forming power, which therefore resulted in less useable light reaching the viewing area. Thus the '801 and '152 projection screens had significant limitations.